1. Field of the Invention
This invention relates to a 2-μm fiber Amplified Spontaneous Emission (ASE) source.
2. Description of the Related Art
Fiber ASE sources are broadband light sources based on the emission properties dictated by the energy structure of dopant ions in the glass host. A pump laser energizes the dopant ions so that spontaneously emitted light from one ion propagates along the fiber where it is amplified by the gain properties of the fiber and emitted as ASE. Light is emitted in both forward and backward directions, relative to pump direction, but the backward ASE, or counter-pumped direction, has a higher quantum efficiency and is selected as the source output. Unlike lasers, ASE sources do not rely on optical feedback, and thus the full-width half-maximum (FWHM) bandwidth of the backward ASE is generally very broad, typically greater than 10 nm.
ASE sources are widely used for test and measurement in such fields as optical spectroscopy of materials, optical component testing, optical coherence tomography, and optical fiber gyroscopes. The incoherent light source enables measurement of insertion loss, crosstalk, bandwidth, polarization dependencies, and other parameters of passive optical components versus wavelength. The most common fiber ASE source comprises a single-mode pump that energizes a length of Er-doped single-mode silica fiber, typically tens of meters, to emit at 1.5 μm. In these ASE sources there is a tradeoff between (1) power and (2) emission bandwidth and spectral stability/purity.
Kyunghwan Oh et al demonstrated the first near 2 micron ASE from Tm3+-doped silica fiber. The core glass of the fiber is a silica composition of at least 93 mol. % silica with 3.9 mol. % Ta2O5, 2.3 mol. % Al2O3, and 0.2 mol. % P2O5 that does not include a glass modifier. (Kyunghwan-Oh; Kilian-A; Reinhart-L; Qin-Zhang; Morse-TF; Weher-PM, “Broadband superfluorescent emission of the 3H4 to 3H6 transition in a Tm-doped multicomponent silicate fiber,” Optics-Letters, 1 Aug. 1994; 19(15): 1131-3). Near 2 micron ASE from double clad Tm3+-doped silica fiber and Tm:Ho doped fluoride fibers using a high power 803 nm diode pump source were demonstrated by Yuen H. Tsang, et al. (Tsang-YH; King-TA; Do-Kyeong-Ko; Jongmin-Lee, “Broadband amplified spontaneous emission double-clad fibre source with central wavelengths near 2 μm,” Journal-of-Modern-Optics. 10 May 2006; 53(7): 991-1001). D shaped Tm-doped silica fiber, rectangular Tm-doped silica fiber, and rectangular Tm:Ho doped ZBLAN fluoride glass fiber were used for the demonstration. The Tm doping concentrations in the Tm only silica fiber are 1.6 wt % and 1.45 wt %.